1. Field of the Invention
The present invention relates to a projection cathode ray tube for projecting the image displayed on a fluorescent surface, on a screen in front of the fluorescent surface through a projection lens as an enlarged image. More particularly, the present invention relates to a projection cathode ray tube which is capable of reducing the deterioration of the light output, with time, by suppressing the browning phenomenon of the glass surface of the face panel and the multilayer optical interference film.
2. Description of the Related Art
In U.S. Pat. No. 4,642,695, filed by the applicant of the present invention is disclosed a method of ameliorating the defect of a projection television set, namely, the poor convergence ratio exhibited when the beams of the respective monochromes emitted from the projection cathode ray tube are received by the projection lens unit.
In an ordinary cathode ray tube, the light emitted from the fluorescent surface assumes a state approximate to what is called perfect diffusion light. However, in a projection television set, among the beams emitted from the fluorescent surface, only the rays having a divergence angle of not more than about .+-.30.degree. are received by the projection lens unit and the other beams are treated as extraneous light. The extraneous light is not only necessary but exerts various deleterious influences. For example, the extraneous light is reflected by a cylindrical mirror of the projection lens unit or the like to become backlight, which lowers the contrast of the projected image. According to the related art disclosed in U.S. Pat. No. 4,642,695, the method is greatly effective for improving the brightness of the image on the screen of a projection television set because not less than 30% of the total light fluxes emitted from a light emitting point of the fluorescent surface is converged into the interior of a conical body having a divergence angle of .+-.30.degree..
In Japanese Patent Laid-Open No. 257043/1985, filed to the Japan Patent Office by the applicant of the present invention, a projection cathode ray tube provided with a multilayer optical interference film composed of a plurality of alternately superimposed layers of high refractive and low refractive index materials disposed between the face panel and the fluorescent surface is disclosed as the concrete example of the above-described related art. As an example of the multilayer optical interference film, a multilayer optical interference film is described which is composed of six alternately superimposed layers of tantalum pentoxide (Ta.sub.2 O.sub.5) as a high refractive index material and silicon dioxide (SiO.sub.2) as a low-refractive index material.
In a conventional projection cathode ray tube provided with a multilayer optical interference film on the inner surface of the face panel, the degree to which the light emitted from the projection cathode ray tube is lowered with the operation time is disadvantageously larger than in a projection cathode ray tube having no optical multilayer interference film. FIG. 2 shows a change in the light output with respect to the operation time which is obtained by continuously operating a projection cathode ray tube emitting green light (G) at a high voltage (accelerating voltage) of 32 KV and a current density on the fluorescent surface of 6 .mu.A cm.sup.-2 (the outer surface of the face panel of the projection cathode ray tube is cooled by a coolant). In FIG. 2, the curve (I) shows the deterioration of the light output of a conventional projection cathode ray tube which has no multilayer optical interference film. It is observed that the light output is lowered to 74% of the initial light output in 7,000 hours. This deterioration will be ascribed to the fact that the luminous efficiency of the phosphor itself is lowered and to the browning phenomenon of the face panel. The ratio of the weights of these causes is considered to be about 50% in the present state of the art.
It is considered that the luminous efficiency of a phosphor is lowered when the luminescent mechanism of the phosphor itself is gradually broken by the energy of the impact of the electron beam and the heat or the X-rays generated thereby. The browning phenomenon is divided into electron beam browning and X-ray browning. Electron beam browning is caused by the reduction of alkaline metal ions such as sodium (Na) ions and potassium (K) ions which constitute the face panel into metals, by the energy produced, when the electron beam which has passed through the gaps of the fluorescent layer directly collides against the inner surface of the face panel. X-ray browning is a kind of solarization and is caused when the energy of the X-rays produced by the electrons which collide against the fluorescent surface or the glass surface at a high speed produces the browning center in the lattice defect in the glass surface of the face panel. If such electron beam browning or X-ray browning is caused, the glass surface of the face panel is tinged with brown and the spectral transmittance is lowered, as shown in the spectral transmittance distribution (b) in comparison with the spectral transmittance distribution (a) before browning in FIG. 3. The lowering of the transmittance becomes larger in the short wavelength range of the visible light.
The curve (II) in FIG. 2 shows the deterioration of the light output of a conventional projection cathode ray tube having a multilayer optical interference film which is composed of a face panel 1, a multilayer optical interference film 2 provided on the inner surface of the face panel and consisting of five alternately superimposed layers of titanium oxide (TiO.sub.2) as a high refractive index material and silicon dioxide (SiO.sub.2) as a low refractive index material, a phosphor layer 3 and a metal back coat 4 overlaid with each other on the optical multilayer interference film, as shown in the sectional view of the face panel and the fluorescent surface of a projection cathode ray tube of FIG. 4. It is observed that the light output is lowered to 63% of the initial light output in 7,000 hours. The deterioration of the light output is much larger than that in the conventional projection cathode ray tube having no multilayer optical interference film (curve (I)). As a result of the analysis of the cause of the deterioration, it has been found that browning is produced on the multilayer optical interference film 2 in addition to the glass surface of the face panel 1. Browning on the multilayer optical interference film 2 is produced on, in particular, the layer of titanium oxide (TiO.sub.2), which is a high refractive index material. It has been found that browning on the titanium oxide layer is caused by the reduction of TiO.sub.2 into TiO.sub.2-x by the energy produced when the electron beam having a high energy, which has passed through the gaps of the phosphor layer 3, rushes into the titanium oxide (TiO.sub.2) layer. As a high refractive index material, an oxide of a metal is ordinarily used. As a result of investigations of various metals which are optically usable, it has been confirmed that a similar browning phenomenon is caused to one degree or another by using any material.